This invention relates to a mechanical pencil in which a pencil lead can be automatically advanced at a rate corresponding to the wear of the lead during writing.
Such a mechanical pencil with automatic lead feed during writing is already known from U.S. Pat. No. 4,180,339 in which the mechanical pencil has a tubular housing accommodating a lead magazine in its rear portion and a chuck or clamping mechanism in its front portion for releasably clamping the lead in response to its movement in its axial direction. To the front end of the housing is connected a conical member for guiding the lead, at the tapered front end of which a small lead tube is slidably mounted. The lead passes through the lead tube until the front end of the former reaches the front end of the latter. As the lead tube always touches the surface of a sheet of paper at its front end during writing with the lead, it is gradually moved rearwardly, by a repulsive force from the surface of the paper during writing, in response to the wear of the lead.
To the rear end of the lead tube is fixedly connected a lead holder for holding the lead in such a manner that it can slidably retract on the lead during writing. A coil spring provided in the conical member is compressed while the lead is being clamped by the clamping mechanism, which can hold the lead to advance the lead under the repulsive force of the spring, the clamping mechanism releasing the lead when the front end of the lead is separated from the paper surface.
In such a construction, the lead tube functions as a guide member for the lead and a sensor for sensing the wear of the lead during writing. For these purposes, the lead tube is adapted to slide through a bore in the front end of the conical member. Therefore, the lead tube cannot hold the lead with sufficient resistance against bending forces exerted on the lead when the mechanical pencil is used at a large angle, especially more than 75 degrees of angle of inclination. In addition, when the bending forces are exerted on the front part of the lead, the lead tube slightly slants with respect to the longitudinal axis of the pencil whereby a part of the outer circumferential surface of the lead tube is apt to stick to the inner surface of the bore to cause obstruction of smooth movement of the lead tube. Accordingly, a user has an unpleasant feel during writing, and the front end of the tube is worn, whereby two sharp corners are formed on respective sides, in the radial direction, of the face of the front end. This often gives rise to tearing of the paper. To avoid wear of the lead tube, a piece of super hard alloy or ruby is often attached to the front end of the lead tube.